The biological activity of a polypeptide often is highly dependent on the specific target epitopes with which it interacts. Examples of polypeptide action include inhibition of an enzyme by targeting the enzyme's active site, blocking a mitogenic factor by competing for its receptor binding site, and targeting site-specific mutations that arise in diseases such as cancer. The development of specifically targeted therapeutics, such as engineered proteins that can discriminate between single mutations, can be essential for targeting disease-associated mutant proteins, without affecting normal proteins. This has proven to be an extremely difficult problem to solve, however, to the extent that some protein mutants have been considered undruggable.